Soft tissue ultrasound


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Soft tissue ultrasound

  1. 1. Soft Tissue Ultrasound: Abscess versus Cellulitis Geoffrey E. Hayden, MD Director of Emergency Ultrasonography Vanderbilt Emergency Medicine Background: • CT is the gold standard for the diagnosis of soft tissue infections, but is of course expensive, time- consuming, sparingly available, may involve contrast agents, radiation exposure, and generally can’t be used to directly guide aspiration/incision • MRI is helpful but not really a viable option through the ED • Plain films are worthless, and reveal soft tissue gas in as few as 40% of cases • U/S is great for diagnosing simple abscesses versus cellulitis and changes management1-5 • Squire et al. report a sensitivity=98%, specificity=88%, PPV=93%, NPV=97% (comparison to PE, clinical exam; conclusion: ED bedside US improves accuracy in the detection of superficial abscesses)6 • U/S in the diagnosis of necrotizing fasciitis is controversial; few studies exist7 • Regarding peritonsillar abscesses (PTA), CT is considered the gold standard; however, U/S compares favorably with sensitivities of ~ 90% and specificities ~ 83-100% Applications: • Confirm cellulitis • Confirm occult abscess not apparent on exam • Localize abscess for drainage • Identify fluid adjacent to deeper fascial planes • Identify a peritonsillar abscess (PTA) Transducers: • 7.5-10+ MHz linear transducers • 3.5-5 MHz curved transducers may play a role in deeper abscesses • ALWAYS USE a probe cover (latex glove is fine) for U/S (protect your probe, protect your patients) • Endovaginal probe (with probe cover, of course) for intra-oral exams (5-10 MHz) Sono Technique: • All fluid collections scanned in 2 planes to define their shape (orthogonal views) • Measurements in 2 planes • Depth measured using markers on display • Use Doppler to r/o vascular structure (don’t be fooled by a pseudoaneurysm!!) • PTA scanning: • May apply topical anesthetic spray to affected tonsil/oropharyngeal swelling • Approach from medial to lateral, choking up on the probe and using the pinky finger to stabilized on the cheek • Scan the entire tonsil for fluid collections, enhancement • Gentle pressure may be applied to assess for “squish sign” • Note the location of the internal carotid artery, which runs anterior to the jugular vein in the carotid sheath; it is usually located posterolateral to the tonsil within 5mm to 25mm of a peritonsillar abscess
  2. 2. Normal sono findings/anatomy: • Subcutaneous tissue generally appears hypoechoic with randomly distributed hyperechoic strands that represent connective tissue • Fascial planes are hyperechoic • Muscle has a characteristic striated appearance in a longitudinal plane • Vascular structures: anechoic • Nerves: stippled appearance • Lymph nodes: classic circular to oval shape, echogenic centers with hypoechoic rims • Normal tonsil has the appearance of a typical lymph node, with a hypoechoic rim and generally echogenic center; may be isoechoic throughout Abnormal sono findings: • Cellulitis: • Diffuse thickening of subcutaneous layer due to edema amidst the fat and connective tissue • Edema evolves as well defined hypoechoic septae between the fat and connective tissue; characteristic “cobble-stone” appearance • Abscess: • Sonographic appearance is quite variable • Ranges from anechoic to irregularly hyperechoic, internal echoes; may find hyperechoic sediment, septae, or even gas • Ranges from round and generally well-defined to irregular, lobulated • Posterior acoustic enhancement may be your only sonographic finding • “Squish sign” with compression: ability to induce motion in the material with palpation/pressure • Necrotizing fasciitis: • Marked thickening of the subcutaneous layer (i.e. cellulitis) • Layer of anechoic fluid measuring >4mm, adjacent to the deep fascia • Subcutaneous gas (acoustic shadowing and reverberation artifact) may be present • Peritonsillar abscess: • Usually appears as a hypoechoic, heterogeneous mass, though appearance may be variable • Commonly see posterior enhancement Pitfalls: • Sometimes difficult to differentiate between interconnected bands of edema fluid and an irregular abscess/pus collection • May miss abscess if isoechoic to surrounding tissue and no posterior enhancement or “squish sign” appreciated • PTA exams may be limited by trismus or gag reflex • Inferiorly located abscesses can be missed by failure to scan in the longitudinal plane Pearls: • Use contralateral side to delineate pathology • Look for areas of echogenicity (suggestive of occult abscess) or a “squish sign” even if no obvious fluid collection
  3. 3. • Use plenty of U/S gel so you limit direct pressure (allows visualization without “hurting” your patient) • Use water bath for hand or foot infections (the water provides the interface, even better than U/S gel!!) 1. Bureau NJ, Chhem RK, Cardinal E. Musculoskeletal infections: US manifestations. Radiographics 1999;19(6):1585-92. 2. Chao HC, Lin SJ, Huang YC, Lin TY. Sonographic evaluation of cellulitis in children. J Ultrasound Med 2000;19(11):743-9. 3. Swartz MN. Clinical practice. Cellulitis. N Engl J Med 2004;350(9):904-12. 4. Tayal VS, Hasan N, Norton HJ, Tomaszewski CA. The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. Acad Emerg Med 2006;13(4):384-8. 5. Vincent LM. Ultrasound of soft tissue abnormalities of the extremities. Radiol Clin North Am 1988;26(1):131-44. 6. Squire BT, Fox JC, Anderson C. ABSCESS: applied bedside sonography for convenient evaluation of superficial soft tissue infections. Acad Emerg Med 2005;12(7):601-6. 7. Yen ZS, Wang HP, Ma HM, Chen SC, Chen WJ. Ultrasonographic screening of clinically-suspected necrotizing fasciitis. Acad Emerg Med 2002;9(12):1448-51.